The present invention relates to a digital phase/frequency control circuit wherein the signal of a voltage-controlled oscillator which includes a varactor diode and whose frequency can be varied by a phase discriminator is applied to the first input of the phase discriminator through a presettable frequency divider serving to select the frequency of the oscillator, wherein the signal of a reference oscillator is applied to the second input of the phase discriminator through a reference-frequency divider, and wherein the phase discriminator has a first output for pulses raising the frequency of the oscillator and a second output for pulses lowering the frequency of the oscillator. A phase/frequency control circuit of this kind is disclosed, for example, in U.S. Pat. No. 4,290,029.
Such phase/frequency control circuits as preferably used for station selection in radio and television sets and are also referred to as "PLL systems" or "frequency synthesis systems". If used in radio and television sets, the voltage-controlled oscillator usually includes at least one varactor diode, to which the control voltage is applied to vary the capacitance and, hence, the frequency of the diode. In tuners working on the superhet principle, the voltage-controlled oscillator is the heterodyne oscillator.
In most cases, such arrangements are provided with an electronic search tuning facility, which searches up or down the frequency band until it finds a signal of sufficient strength. Particularly in such search modes, but also when the user switches between widely separated channels of a frequency band, the control voltage may reach the breakdown-voltage region of the varactor diodes. In this region, the varactor diodes, which previously had a high series resistance, rapidly become low-resistance elements and, thus, heavily damp the oscillator, so that oscillator may stop. On the other hand, however, it is also possible that the control voltage becomes so small that the varactor diodes will be driven into their forward-biased regions. Then, too, the oscillator will be heavily damped.
In both cases, the phase/frequency control circuit cannot find its way out of the respective unwanted mode by itself.